1. Field of the Invention
This invention relates to an optical head for use in an optical information recording-reproducing apparatus such as a magneto-optical disk apparatus, and particulary to a plural-beam optical head for imaging a plurality of light beams on an optical information recording medium. The optical head of the present invention can be applied as an optical head for effecting at least two of the erasing, recording and reproduction of information at a time.
2. Related Background Art
As an optical head for recording and reproducing a magneto-optical signal by the use of a double-refractive crystal prism, there is one as shown in FIG. 1 of the accompanying drawings. In this figure, a servo signal detection system is omitted. In FIG. 1, a light beam 50, which is emitted from a semiconductor laser 1 and collimated by a collimator lens 2, is transmitted through a beam splitter 3 with a beam shaping portion, and is imaged on a magneto-optical disk 5 by an objective lens 4, whereby the erasing, recording or reproduction of information is effected. The light beam 50 that is reflected by the magneto-optical disk 5 is again collimated by the objective lens 4 and is now reflected by the beam splitter 3 with a beam shaping portion, and is directed to a signal reproducing optical system. The signal reproducing optical system comprises a half wavelength plate 6, a double-refractive crystal prism 7, a condensing lens 8 and an RF sensor 9. The light beam reflected by the beam splitter 3 with a beam shaping portion has its direction of polarization rotated by about 45.degree. by the half wavelength plate 6 and is directed to the double-refractive crystal prism 7. The double-refractive crystal prism 7 comprises prisms 7a and 7b whose crystal axes are orthogonal to each other (the crystal axis of the prism 7a being perpendicular to the plane of the drawing sheet of FIG. 1, and the crystal axis of the prism 7b being perpendicular to the incident light beam in the plane of the drawing sheet of FIG. 1), and the cemented surface thereof is perpendicular to the plane of the drawing sheet of FIG. 1 and forms approximately 45.degree. with respect to the direction of incidence of the light beam, and separates the incident light beam into two light beams whose directions of polarization are orthogonal to each other, in the plane of the drawing sheet of FIG. 1. Where the prisms 7a and 7b each comprise rock crystal, the two light beams form an angle of about 1.degree. therebetween. The light beam incident on the double-refractive crystal prism 7 is separated into two light beams 51 and 52 whose directions of polarization are orthogonal to each other, in the plane of the drawing sheet of FIG. 1. The separated two light beams 51 and 52 are Condensed on the RF sensor 9 by the condensing lens 8. The RF sensor 9 comprises at least two light receiving portions, and the output thereof makes a differential signal and thereby becomes a magneto-optical signal.
Now, it has heretofore been proposed to 10 insert a diffraction grating into a system as shown in FIG. 1 in order to effect recording and reproduction at one time (for example, Japanese Laid-Open Patent Application No. 64-82384). FIG. 2 of the accompanying drawings shows a system having a diffraction grating inserted into the system of FIG. 1. A plurality of light beams 50, 60 and 70 are made by a diffraction grating 10. These light beams travel along substantially the same path as that previously described and each of them is separated by the double-refractive crystal prism 7 into light beams whose directions of polarization are orthogonal to each other (the light beam 50 is separated into light beams 51 and 52, the light beam 60 is separated into light beams 61 and 62, and the light beam 70 is separated into light beams 71 and 72), and these separated light beams are condensed on an RF sensor 12. When for example, the light beam making a relatively preceding spot on the magneto-optical disk 5 with respect to the direction of rotation thereof is designated as 50 and the light beam making a succeeding spot is designated as 60, recording is effected by a spot corresponding to the light beam 50 and at the same time, reproduction is effected by a spot corresponding to the light beam 60, and from the differential signal of the output from the RF sensor 12 which corresponds to the light beams 61 and 62, there is obtained a reproduction signal as a magneto-optical signal.
Now, in the example of the prior art as shown in FIG. 2, the direction of arrangement of the plurality of beams entering the double-refractive crystal prism and the direction of light beam separation by the double-refractive crystal prism coincide with each other (in FIG. 2, in the plane of the drawing sheet thereof) and therefore, as shown in FIG. 3 of the accompanying drawings which is a partial enlarged perspective view, a plurality of spots are arranged on substantially the same straight line on the RF sensor 12. Accordingly, to receive the plurality of spots without them interfering with one another, the condensing lens 8 must be set so that in FIG. 3, not only the spots corresponding to the light beams 51, 61 and 62 may not interfere with one another and further the spots corresponding to the light beams 52, 71 and 72 may not interfere with one another, but also the spots corresponding to the light beams 62 and 71 may not interfere with each other, and this has led to the problem that the optical system unavoidably becomes bulky. Such a circumstance also holds true when the plurality of beams are made by a plurality of lasers, a laser array or a double-refractive crystal. If the circumstance is restricted to a case where the plurality of beams are made by a diffraction grating, although in FIG. 2, there are shown 0-order diffracted light (corresponding to the light beam 50) and .+-.1st-order diffracted lights (corresponding to the light beams 60 and 70), diffracted lights of still higher orders also become non-negligible due to a slight manufacturing error or the like in the diffraction grating. This in turn has led to problem that the interference between spots on the RF sensor becomes unavoidable.